1. Field of the Invention
This invention relates to tubing retrievable surface controlled subsurface safety valves for controlling flow at a subsurface location in a well.
2. The Prior Art
Surface controlled subsurface safety valves have been used to control flow in a well at a subsurface location for some time. Some such valves are tubing retrievable. (See U.S. Pat. Nos. 3,860,066; 3,007,669 and 2,894,715.) The valve forms an integral portion of the tubing string. In the event of a malfunction, replacement of the valve requires retrieval of the tubing string and placement of a new valve therein.
To avoid retrieval of the tubing string and replacement of the malfunctioned tubing retrievable valve, some tubing retrievable valves are adapted to be locked open. Thereafter, a second wire line or pumpdown retrievable valve is landed in the tubing string and that second valve controls subsurface fluid flow. (See U.S. Pat. Nos. 3,696,868; 3,762,471; 3,856,083; 3,870,102; and 3,882,935.) To reduce the complexity of the well's hydraulic control system, it is desirable to operate the second valve utilizing the same conduits that had been used to communicate control fluid to the malfunctioned tubing retrievable valve. However, if the seals which render the tubing retrievable valve control pressure responsive fail, remedial operations, such as installing a wireline or pumpdown retrievable valve, could prove ineffective. First, if leakage past the seals is severe, the hydraulic control system will not be able to control the second valve. Seal leakage prevents the obtainment of sufficient pressure to open the second valve and maintain it opened. Second, high pressure well fluids may escape from the tubing string bore past the failed seals. A well blowout could thereby be initiated.
The well's tubing string may be equipped with a special landing nipple and a control conduit terminating at the landing nipple. The landing nipple may include a sleeve valve to control fluid communication from the control conduit. A wireline retrievable valve may be landed, locked, and sealed in the landing nipple and simultaneously the sleeve valve is shifted to permit control fluid from the control conduit to be effective to control the valve. (See U.S. Pat. No. 3,990,511.)
A sleeve valve (as disclosed in the Composite Catalog of Oil Field Equipment and Services, 1976-77 Ed. pp. 4572 and 4574) may be shifted to isolate the operating seals of a tubing retrievable surface controlled subsurface safety valve. However, for a subsurface safety valve structured in accordance with that disclosure, separate trips and different operating tools are required to lock the safety valve in an open position, block passage of control fluid to the operating seals, and land a secondary valve in the tubing string bore.